Effect of spinal cord stimulation on sensory nerve conduction threshold functional measures.

Background. Spinal cord stimulation (SCS) is being used with increasing frequency in the treatment of various chronic pain conditions. There is a paucity of reliable outcome data regarding changes in pain tolerance and peripheral sensory nerve function. The automated electrodiagnostic neuroselective sensory Nerve Conduction Threshold (sNCT) test measures painless current perception thresholds (CPTs) and atraumatic pain tolerance thresholds (PTTs). The ability of the sNCT test to independently evaluate small and large fiber function may have particular relevance for evaluating response to SCS. Methods/Results. Sixteen patients with implanted SCS systems and lower extremity neuropathic pain of greater than 6-months duration were tested using a standardized protocol, pre- and post-SCS. CPT and PTT measures (Neurometer, CPT/C Neurotron, Inc. Baltimore, MD) were obtained from the distal phalange of the most symptomatic extremity and at an ipsilateral asymptomatic control site. Only CPTs at the symptomatic site (2000 Hz only) and at the control site (5 Hz only) reached statistical significance. Changes in CPTs at other frequencies, and changes in PTTs at all frequencies (symptomatic and control sites) were not statistically significant. Conclusion. The results of this study appear to substantiate the postulates that both segmental and suprasegmental effects are involved in SCS-mediated analgesia. SCS modulates segmental large afferent fiber input as reflected by a statistically significant increase in large fiber CPTs (2000 Hz) at the symptomatic site post-SCS. A statistically significant increase in small fiber (5 Hz) CPTs at the control site suggests a central sensory (suprasegmental) modulating effect on nociceptive fiber activity. sNCT testing provided reliable outcome data for evaluating response to SCS.